Etymology

Always derived from ON, cp. ODan banke ‘hill, rising ground’ (Dan, Norw banke), Sw dial bank ‘bank of clouds’.  This word is usually regarded as etymologically identical with OIcel bakki ‘bank (of river, lake), shore; slope, escarpment; edge (etc.)’, Far bakki, ODan bakkæ (Dan, Norw bakke), Sw backe, i.e. representing a variant without assimilation of /nk/ > /kk/ (for doubts, see e.g. de Vries, Bj.). ODan banke presumes a PGmc wk. masc. *bank-ōn (formed on the same root *bank- as the i-stem *bank-iz, as in OE benc ‘bench’) and perhaps continued outside of Scandinavia by MLG bank ‘bank, shore’ or, more debatably, OE hobanca (see OED). VEPN also entertains possible influence from ODan banke on the sense of the English n. benc ‘bank or ridge of earth’, as indicated by Bj. (230n). 

PGmc Ancestor

*bank-ōn

Proposed ON Etymon (OIcel representative)

bakki ‘bank (of river, lake), shore; slope, escarpment; edge (etc.)’
(ONP bakki (1) (sb.))

Other Scandinavian Reflexes

Far bakki, Icel bakki, Norw bakke, banke, ODan bakkæ, bakke, banke, Dan bakke, Sw backe, Sw dial bank

OE Cognate

cp. benc ‘bench’ 

Phonological and morphological markers

Summary category

C1abc

Attestation

Common and widespread  by the later 14c., but recorded earlier only in Orrm (for a detailed discussion of distribution and semantics, see Hug, esp. pp. 148-9, 155–6, 185, 188–90.) Characteristically N and E in place-names: see VEPN, and further Elliott 1984: 75, 79–80, 87–8.

Occurrences in the Gersum Corpus

Gaw 14, 511, 700, etc.; Pe 102, 106, 196 etc.; Cl 86, 363, 379 etc.; Erk 32; WA 2441, 4189, 5243

<bonk> appears to have been used as pl. for at Cl 379 (see Anderson's note).

Bibliography

MED bank(e (n.1) , OED bank (n.1) , HTOED and HTOED , Dance bonk, Bj. 230, de Vries bakki (1), Mag. bakki (1), Nielsen bakke (I); banke (I), Hellquist backe, Falk-Torp bakke (I); banke, Torp NNeO bakke, Orel *bankōn, Kroonen *bankan-, AEW banca, VEPN banke